M4 is a short, transcribed repeat sequence interspersed with single-copy DNA in the Dictyostelium genome. I have isolated sets of cloned mRNAs (cDNAs) and their corresponding genomic sequences which contain M4 repeat elements covalently linked to different single-copy sequences. Sequence analyses of these cDNA and genomic fragments indicate that the common element is AAC/GTT. This sequence is asymmetrically expressed; only AAC sequences are represented in mRNA. The M4 sequences are expressed at approximately 5-fold greater relative level of expression in cells which are developing than those in vegetative growth. Individual M4 mRNAs possess a specific pattern of expression which are similar to the total M4 population. Preliminary results indicate that an approximately 15,000 dalton protein interacts with the AAC/GTT sequences. Our results may suggest that the M4 sequences are involved in the developmental regulation of a gene family during Dictyostelium development. I have identified an independent transcription unit whose expression decreases as development proceeds. Analyses of its kinetics of expression indicate that it is negatively effected by high levels of cyclic AMP which occur early during the developmental cycle of Dictyostelium.